Micro- and nanostructures can be produced in a substrate by etching away selected parts of the surface of the substrate. These parts of the surface of the substrate are contacted with an etchant which is capable of reacting in an etching manner with the substrate material. As a rule, the substrate is before etching coated with a layer of etch-protecting material (resist) which reacts slowly, if at all, with the etchant. The etch-protecting layer is often produced in a multistage process comprising a coating step, in which the etch-protecting material is applied over the entire substrate surface, and an exposing step, in which a pattern of exposed portions is produced in the etch-protecting material applied to the substrate. The exposing step can be accomplished by the applied etch-protecting material (photo resist) being radiated through a mask and subsequently developed for exposing of either the radiated or unradiated portions thereof.
During the etching procedure, dissolution and/or mechanical removal of the substrate material is accomplished in the exposed portions. The material-dissolving etching methods can be divided into wet and dry techniques. In wet material-dissolving etching methods a liquid etchant is used. In chemical wet etching the etchant reacts spontaneously with the substrate material, and in electrochemical wet etching the etchant reacts by electrochemical reactions at the surface of the substrate material when the etchant, with applied voltage, carries a current.
In wet etching methods, and also in some dry etching methods, such as plasma etching, the material-dissolving surface reactions which are intended to occur merely on the exposed portions of the substrate, also have a certain tendency to occur in the periphery of the exposed portions, and also under the etch-protecting layer, and thus give rise to so-called underetching. Such underetching restricts the possibilities of producing narrow and deep structures, e.g. to arrange conductors close to each other. Although an increased duration of the etching procedure results in an increased depth of the etched structures, it also results at the same time in an increased tendency to underetching.
Even if dry etching methods are preferred in this respect as they produce a result with less under-cut, they are not as fast as wet etching methods.
Examples on dry etching are given in U.S. Pat. Nos. 5,962,346, 5,176,792, 5,837,616 and JP 3 082 120. Thus, when the etching is to be performed at a high rate, wet etching methods are preferred due to their higher capability to etch at high rates. This is primarily due to the fact that transportation of etchant towards the substrate as well as transportation of material away from the substrate is faster in a fluid than in a plasma during wet etching.
U.S. Pat. No. 4,877,480 discloses a method for etching a substrate by removing material by means of laser. For cooling of the substrate during radiation, the substrate is immersed in a liquid which is capable of etching and which is activated by radiation. During the etching procedure, the removing etching effect of the laser light is thus assisted by the material-dissolving etching effect of the liquid. It is said that etching of smaller structures than normal is made possible.
U.S. Pat. No. 5,279,702 discloses a technique for chemical wet etching of a substrate. The substrate which is coated with a patterned resist layer, is immersed in a spontaneously etching etchant. This etchant contains a passivating substance which spontaneously forms an etch-protecting compound on the exposed portions of the substrate, whereby these portions become unreactive to the etchant. By radiation of the substrate perpendicular to its surface, the etch-protecting compound is removed in the direction of the radiation so that etching can proceed in exposed portions of the substrate. However, the etch-protecting compound prevents continued etching at the periphery of the exposed portions, which is protected from radiation by the superposed etch-protecting layer. During the etching procedure, a barrier thus forms on the side walls of the etching structure, thereby minimizing underetching.
A limitation of the above techniques is that the substrate must be radiated during the entire etching procedure to enable etching. Such radiation can in many cases be difficult to provide.